Acetylene recovery



Patented Mar. 2, 1954 UNITED STATES- PATIENT" F FlC-E-- ACETYLENE RECOVERY William. Smith Dorsey, Fullerton, Calif., assignor toUnion, Oil Company,.of California, Los Angeles,. Calif;, a corporation ofCalifornia No Drawing, Application January 21, 1952, Serial No. 267,494

9 claims, ,(01. 183 -115) This, invention relates to an, improved method for-.recovering acetylenefrom gas mixtures, and in particular concerns a process wherein acetylene-containing gasesarecontacted or scrubbed with. certain organic liquids in whichacetylene is highly soluble. It further concerns new and useful solutions of acetylene. dissolved in such.

10 percent by volume and is usually of theorder.

of 3 to ,6 per cent by volume. Various methods for recovering the acetylene in substantially pure form from the product gashave been proposed.

The majority of such methods involve scrubbing the gas with-a selective solvent for the acetylene m on nt va d t re te in ..o ili the acetylene ,away from the solvent. When suchmethod is applied to the treatment of large volumes of gas containing only. small amounts of acetylene, it is essential that thesolvent, in addition to having high solvent power .for acetylene,.also.have as, high a boiling point as possi; ble in order, to minimize solvent loss. Thus,

acetone, despite its high solvent power for acetylene, cannot economicallybe employed f or .35 becauseof its ,normallyhigh vapor, pressure.

scrubbing acetylene out of dilute gas mixtures Other, criteria for. suitable solvents include 'low viscosity, chemical stability, and inertness with respect to the components of the gas mixture to be treated. A wide variety of compounds have been proposed for use as solvents tor the recovery of .acetylene, e. g., glycolethers and esters, gyl- 5 colether-esters, monoand polyketones, polybasic acid esters, aliphaticlactones, carbonate esters, polyglycols and their ethers and esters, dialkylformamides, tetralkyl ureas, etc.- However, all of such previously proposed solvents have unduly high vapor pressures or leave much tobe desired in the Way of solvent power for acetylene, and many lack selectivity with respect to hydrocarbons other than acetylene.

It is accordingly an object of the present in their somewhat greater solvent power foracetyvention to provide solvents better adapted than those heretofore known for use in recovering acetylene from mixed gases.

Another object-is to provide compounds-hav ing. exceptionally highsolvent power for acety lene coupled with low ,vapor pressure and low' solvent power for thegaseswithwhich acet lene is normally contaminated.

Another object is to provide concentratedsolutions of acetylene in organic solvents A further object is ,to. provide. an, improved method for recovering acetylene from gas--mix-' .tures, particularly .those obtained in hydrocarbon cracking or. partial oxidation processes.

Other. objects will bev apparent frointhe fol:

lowing description of the invention, and various,

advantages not specifically referred,..to,, herein-.

will be apparent to thoseskilled-in theart upon-,-

employment of the inventionin practice",

I have now found-that the aboveandrelated objects may be realized through the use .of the. I formyleor acetyl derivatives of certain hetero cyclic compounds as solvents for. ;acetylene, More particularly, I have. found that theQ N- formyl and N-acetyl derivatives of certain nitrogen-containing heterocyclic .comppunde, have... very high solvent powerfor acetylene .coupled with very lowvapor pressure at ordinar p ess.

sures, ..and are highly selectivewith respect/to,

a wide variety of gases, including v those with; which acetylene is ordinarily associated Such; solvents areinert with respect'to acetylene and. the gases normally, associated. therewith and are relatively inexpensive. The invention thus consists in the method. for recovering acetylene; from gas mixtures .containing the-same which comprisesscrubbing said mixtures witha liquid essentially comprising at least one .member. of,

the hereindefined class of chemical compounds,,.

whereby. the acetylene is selectively. dissolyedior absorbed in said liquid, andin solutions of .acety lene dissolved in said liquid. The heterocyclic compounds which are employedaasacetylene, solvents; in accordance with the inventiomare N-formyl-pyrrolidine, N-acetylpyrrodlidine, N -formyl-piperidine and N-acet-- yl-piperidine. 1 Of these compoundsthe-two,pyr-; rolidine derivatives are preferred .by -reasoniof lene and their somewhatbetter selectivity With respect to other hydrocarbon gases. 1 All of suc h compounds are high-boiling, relatively mobile liquids which are inert with respect to acetylene and the gases with which it is usually contaminated. In-the practicaof the present invention thesecompounds are usually employed v in substantial ly pure form, but if desiredthey-may be employed in admixture with one another or in combination with another liquid which may or may not be a solvent for acetylene.

The formyl compounds of the present class are conveniently prepared simply by adding formic acid to the heterocyclic compound, and heating the mixture gently until reaction is complete. Usually a small excess of the formic ene is usually one of the most difflcult to separate by reason of its analagous unsaturated character and molecular weight. The following table presents a comparison of the solubilities of acetylene and ethylene in the solvents of the present class, and the ratio of acetylenesolubility to ethylene solubility:' i I acid is employed, and if desired the reaction may be carried out in the presence of an inert reaction solvent. Upon completion of the reaction the product may be isolated and purified by fractional distillation. The acetyl compounds are prepared in the same general manner except that acetic anhydride or an acetyl halide is substituted for the formic acid.

The following example is illustrative of the preparation of the compounds of the present class, but is not to be construed as limiting the invention:

Approximately 43.7 parts by weight (0.95 mole) of formic acid are slowly added with stirring to 66 parts by weight (0.93 mole) of pyrrolidine. An exothermic reaction occurrs accompanied by a rise in temperature to about 125 C. Upon completion of the addition of the formic acid, the reaction mixture is heated at about 125 C. for about 20 minutes, after which it is fractionally distilled to recover the N-formyl-pyrrolidine product. The latter is obtained as a substantially colorless liquid distilling at 84.4=-85.0 C. under 8 mm. pressure and having a density of about 1.0880 g./ml. at 24 C. N-formyl-piperidine is prepared in a similar manner. It distills at about 81.7? C. under 5 mm. pressure, and has a density of about 1.0397 g./ml. at 24 C. N-acetyl-pyrrolidine is prepared by substituting acetic anhydride for the formic acid in the foregoing synthesis, and is found to ,distill at about 739 C. under 5 mm. pressure and to have a density of about 1.009 g./ml. at 24 C. N-acetyl-piperidine is similarly prepared, and is found to distill at about 80.6-81.1 C. under 5 mm. pressure and to have a density of about 1.010 g./ml. at 24 C.

The following table presents a comparison of the solubility of acetylene in the solvents of the present class with the solubility in a number of other compounds which have been suggested for use as acetylene solvents:

. 02H: Solubility, Solvent Boiling Point, C. VOL/Vol. at 1 atm.

N-Folmyl-pylrolidinc 84.4-85.0 at 8 mm. 21.4 at 23.8 C. N-Acetyl-pyrrolidine 73 9 at 5 mm 24.4 at 23.7 C. N-Formyl-piperidlne 81 7 at 5 mm 15.0 at 24.0 C. N-Acetyl-piperidine 80 68l.l at 5 mm 20.8 at 23.8 C. Diethyl Oxalate... 185 at 760 mm 1. 12 at 20 C. Butyrolactone 206 at 760 mm at 27 C. 1entanedione-2A 139 at 746 mm 7.5 at 30 C. D'ulnethlyl tetraethylene 276 at 750 mm" 11.4

g yco Acetone 56.5 at 760 mm 13.1 at 25 C.

In addition to their high boiling points and X-- cellent solvent power for acetylene, the compounds of the present class are very poor solvents for the gases with which acetylene is usually contaminated. Of such contaminating gases, ethyl As will be apparent from the foregoing, the properties of the compounds of the present class render these compounds exceptionally well adapted for use as solvents for extracting acetylene from gas mixtures comprising only relatively small amounts of the same. Such extraction process consists essentially in scrubbing the mixed gas with the solvent, whereby the acetylene is preferentially absorbed, separating the rich solventfrcm the non-absorbed gas, and thereafter treating the rich solvent to recover the acetylene therefrom. Theinitial scrubbing step may be carried out in the conventional manner in conventional equipment, e. g., a bubble-cap or packed absorption tower operating, on the countercurrent principle. The operation may be carried out at atmospheric, subatmospheric or superatmospheric pressure 'with' superatmospheric, pressures of thev order of about 20 to about 200 lbs/sq. inqbeing particularly suitable. The solu tion of acetylene in the solvents of the present class closely follows Henrys law in that the weight of acetylene dissolved'by a given weight of solvent increases substantially linearly with the partial pressure of the acetylene. Since the solubilitylof'the ,acetylen'e' yariesi indirectly with temperature, the absorption or scrubbing operation is advantageously carried out at as lowatemperature as can conveniently or economically be maintained. When the present process is applied to the recovery .of acetylenev from the efiiuent gas of a high temperaturehydrocarbon cracking or partial oxidation process, such gas is preferably first cooled to a temperature below C. to condense out the water vapor, after which it is passed to the absorption or scrubbing step of the present process with'or Without compression. As a general rule, it is preferred to effect the absorption or scrubbing step at an absolute pressure between'about 50 and about lbs./sq. in. and at a temperature between about 10 and about 100 C.

Desorption of the acetylene from the rich sol vent may likewise be effected in any of the conventional ways employing, conventional equipment. Since the desorption operation is the reverse of that carried out in the absorption or scrubbing step, it is favored by elevated temper atures and/or low partial pressures of acetylene. Preferably, both of the se factors are combined, 1. e., the richsolvent is heated under partial vacuum untilsubstantially .all of the acetylene is driven off. Alternatively, it may be gas stripped at ordinary or elevated temperatures. The stripping gas should be one which can readily be separated from the acetylene by condensation or other the solvents of the present class, the vapors of A typical product gas obtained from the high temperature partial oxidation of natural gas has the following composition on a dry basis:

Percent by volume Acetylene 3.6 Nitrogen 42.8 Hydrogen 42.0 Carbon dioxide 0.5 Carbon monoxide 5.7 Ethylene 0.4 Methane 5.0

This gas is compressed to about 90 lbs/sq. in., and is passed upwardly through a bubble-cap absorption tower countercurrent to a descending stream of N-formyl-pyrrolidine. The liquid-togas ratio is maintained at about 0.70 1b./cu. ft. The acetylene-rich solvent withdrawn from the bottom of the tower is passed to a distillation column wherein it is heated to about 50 C. under mm. pressure to drive off the dissolved acetylene. The lean solvent withdrawn from the bottom of the column is cooled and recycled back to the top of the absorption tower. The gas taken from the top of the distillation column contains about 95 per cent of acetylene. If desired, it may be further purified by a second solvent extraction operation carried out with the same or a difierent acetylene solvent.

While the invention has been described above primarily as a new and useful process for recovering acetylene from gaseous mixtures comprising the same, it will be apparent that it further comprises new solutions of acetylene which are useful in the art. For example, acetylene may advantageously be stored and/or shipped in the form of a solution in one or a mixture of the present solvents in the same way as acetone solutions of acetylene are commonly employed for the same purpose. As will be apparent from the table above, the solubility of acetylene in N-acetyl-pyrrolidine, for example, is almost twice as great as the solubility of acetylene in acetone. Accordingly, shipping a given volume of acetylene in the form of a solution in N-acetyl-pyrrolidine involves the transportation of only about half as much solution as is required when acetone is employed as the solvent.

Other modes of applying the principle of my invention may be employed instead of those explained, change being made as regards the steps or materials employed provided the methods or compositions stated by any of the following claims, or the equivalent of such stated methods or compositions, be employed or obtained.

I, therefore, particularly point out and distinctly claim as my invention:

1. The process for recovering acetylene from a gas containing the same in admixture with ethylene which comprises scrubbing said gas with a solvent essentially comprising a compound selected from the class consisting of N-formylpyrrolidine, N-acetyl-pyrrolidine, N-formyl-piperidine, and N-acetyl piperidine.

2. The process of claim 1 wherein the solvent essentially comprises N-formyl-pyrrolidine.

3. The process of claim 1 wherein the solvent essentially comprises N-acetyl-pyrrolidine.

4. The process of claim 1 wherein the solvent essentially comprises N-formyl-piperidine.

5. The process of claim 1 wherein the solvent essentially comprises N -acetyl-piperidine.

6. The process for recovering acetylene from a gas containing the same in admixture with ethylene which comprises scrubbing said gas with a solvent essentially comprising a compound selected from the class consisting of N-formylpyrrolidine, N-acetyl-pyrrolidine, N-formyl-piperidine, and N-acetyl piperidine, said scrubbing being carried out at a pressure between about 20 and about 200 lbs./sq.in. and at a temperature between about 10 C. and about C., separating the solvent containing dissolved acetylene from non-absorbed gases, and recovering acetylene from the separated solvent.

7. The process of claim 6 wherein the acetylene is recovered from the separated solvent by heating said solvent under partial vacuum.

8. The process of claim 6 wherein the acetylene is recovered from the separated solvent by stripping said solvent with a condensible vapor of a saturated hydrocarbon.

9. A composition of matter essentially comprising a solution of acetylene in a solvent essentially comprising a compound selected from the class consisting of N-formyl-pyrrolidine, N-acetylpyrrolidine, N-formyl-piperdine, and N-acetylpiperidine.

WILLIAM SMITH DORSEY.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,900,655 Metzger Mar. '7, 1933 2,371,908 Morris et a1 Mar. 20, 1945 2,405,693 Hamill et al Aug. 13, 1946 2,599,649 Lorenz June 10, 1952 FOREIGN PATENTS Number Country Date 29,750 Great Britain Dec. 24, 1897 

1. THE PROCESS FOR RECOVERING ACETYLENE FROM A GAS CONTAINING THE SAME IN ADMIXTURE WITH ETHYLENE WHICH COMPRISES SRCUBBING SAID GAS WITH A SOLVENT ESSENTIALLY COMPRISING A COMPOUND SELECTED FROM THE CLASS CONSISTING OF N-FORMYLPYRROLIDINE, N-ACETYL-PYRROLIDINE, N-FORMYL-PIPERIDINE, AND N-ACETYL PIPERIDINE. 